Electrolytic process for stripping copper



March 11, 1952 R. PEASLEE 2,588,566

ELECTROLYTIC PROCESS FOR STRIPPING COPPER Filed Feb. 4, 1948 ALTERNATINGl0 cunazm INVENTOR RDEERT L. F'EAELEE.

BY Y m AGML ATTORNEY Patented Mar. 11, 1952 1 1" ELECTROLYTIC PROCESSFOR STRIPPING COPPER Robert L. Peaslee, Packanack Lake, N. J assignor toCurtiss-Wright Corporation, a corporation of Delaware ApplicationFebruary 4, 1948, Serial No. 6,159

2 Claims. (01. 204-146) This invention relates to the removal of copperand other metals from the surfaces of steel parts or other bodies and isparticularly directed to a novel method for effecting such removal. Thepresent invention constitutes an improvement over that disclosed inco-pending application Serial Number 590,644, filed April 27, 1947, inthe name of R. L. Peaslee and T. K. Mooney (now abandoned).

In order to harden portions of the surfaces of steel parts, it isconventional practice to carburize said surface portions by exposure ofsaid surface portions to a carbonaceous atmosphere at elevatedtemperatures. The surface portions of said steel parts which are not tobe carburized are protected against this heat treatment by platingcopper over said surface portions. After the carburizing treatment, thecopper plate is stripped or removed from the steel parts. It is anobject of this invention to provide a novel and simple method forstripping or removing copper plate from steel parts without injuring thesteel parts in any way.

Specifically the method of the present invention comprises the steps ofimmersing a pair of copper plated steel or other metallic parts in abath of an ammonium salt and passing an alternating electric currentthrough said bath with said parts comprising opposite electrodes forsaid current. Said bath preferably comprises a solution of ammoniumcarbonate. However, various other ammonium salts may be used such asammonium acetate, ammonium sulfate, ammonium phosphate, ammoniumoxalate, ammonium nitrate, and ammonium bifluoride. In fact the use of asolution of almost any ammonium compound, whether organic or inorganicis within the scope of this invention. A bath of ammonium carbonate ispreferred because of its fast removal of copper and because it does notetch or otherwise attack the steel. With a bath of ammonium carbonate,an excess of oxygen and ammonia are preferably maintained within thebath.

Other objects of the invention will become apparent upon reading theannexed detailed description in connection with the drawing in which:

Figure 1 is a schematic view of the apparatus employed in carrying outthe novel stripping process; and

Figure 2 is a sectional view taken along line 2--2 of Figure 1.

Referring to the drawing, a tank l0, illustrated by dot-and-dash lines,is filled with a bath comprising a solution of ammonium carbonate,(NHOzCOa. The steel or other metallic parts,

ed across the tank In on a pair of terminals 20 mounted on said tank butelectrically insulated therefrom. Similarly the rod I6 is adapted to besupported across the tank on a pair of terminals 22 mounted on said tankbut electrically insulated therefrom.

When copper plated steel parts, such as gears I2, are immersed in anammonium carbonate solution, said solution acts to remove copper fromsaid steel parts. This action takes place even though no electriccurrent is passed through the solution. As disclosed in saidaforementioned application, if direct current is passed through thesolution with a copper plated steel part comprising the positiveelectrode or anode for said direct current, the rate of removal of thecopper from said part is greatly increased. In using direct current, itwas found that copper does not plate out or deposit on the negativeelectrode or cathode as might be expected. This fact indicated thatalternating current could be used instead of direct current. Upon tryingconventional cycle alternating current, applicant discovered that for arate of alternating current flow equal to a given rate of direct currentfiow, copper was removed twice as fast with alternating current ascompared with direct current. In addition, upon suspending a pair ofcopper plated steel parts in an ammonium carbonate solution andconnecting said parts to opposite terminals of a source of alternatingcurrent, it was further discovered that copper was removed approximatelytwice as the periodical reversal of alternating current prevents theformation of some sort of insulating or inactive layer around the copperplated steel workpieces, so that although with alternating currentcopper is being removed electrolytically from a workpiece only duringthe periods that said workpiece is the anode, its rate of removal is sofast during said periods that its average rate of removal is twice therate of removal with direct current of the same magnitude as saidalternating current.

As illustrated in Figure 1, the rods and I6. are connected to theopposite terminals of asource of conventional alternating current 24,having a frequency of 60 cycles per second, by a switch 26 and wires 21.When the switch 28 is closed, copper is stripped or removed from theworkpiece I2 at the aforementioned fast rate. That is, with alternatingcurrent, copper is removed from both workpieces 12 at twice the rate itwould be removed from one of said workpieces if the rods l4 and It wereconnected to a source of direct current with said direct current havingthe same rate of flow as the alternating current.

The rate of removal of copper from the workpieces l2, increases withincrease of the rate of alternating current flow. As long as theworkpieces are not being burned by arcing of the electric current, thereis no upper limit to the rate of current flow. Preferably the range ofthe density of the current flow through the bath is from 10 to 25amperes per square foot.

The removal of copper is facilitated byaerating the bath with air. Inaddition, in order to check depletion of the bath, ammonia gas, NHs, isintroduced therein. As illustrated, air and ammonia gas are introducedinto the bath at the bottom of the tank l through a suitable aerator 28.The aerator 28 comprises a plurality of pipes 30 extending across thebottom of the tank l0 and each of the pipes 30 is provided with aplurality of holes 32 along its length. Air and ammonia gas are suppliedto the aerator through a pipe 34 which in turn is connected to air andammonia supply pipes 36 and 38 respectively. In this way, duringoperation of the process, the bath is aerated with fine bubbles of airand ammonia. The holes 32 should be quite small and closely spaced inorder to obtain a large amount of aeration with as small an amount ofgas as possible. It has been found satisfactory to use a numberseventy-one size drill for forming the holes 26.

The air supply pipe 36 is provided with a valve 40 having a valve stem42 comprising a plunger of a solenoid 44. When energized, the solenoid44 moves the valve stem 32 to open the valve. Upon de-energization ofsaid solenoid, the valve 40 is arranged to closeas for example by aspring. The electric circuit for the solenoid 44 includes a source ofelectric energy 46 and a pair of serially connected electric switches 48and 50. The switch 48 is normally open and. is adapted to be closed bythe rod [4 when said rod is placed in position across its terminals 58.Similarly the switch 50 is normally open and is arranged to be closed bythe rod it when said rod is placed in position across its terminals 22.With this construction, unless both electrode rods 14 and I6 aresupported in position across the tank ID, the air valve 40 is closed.When both electrode rods I4 and I6 are supported in position across thetank It], both switches 48 and 50 are closed thereby effectingenergization of the solenoid 44 and opening the air valve 40. In thisway, when the tank I0 is not in use, the air valve 40 is automaticallyclosed thereby preventing removal of the free ammonia within thesolution in the tank l0 by the drag-out effect of the air passingthrough the solution within the tank.

Ammonia gas is very soluble. Accordingly, if the air flow through thepipe 34 were completely shut off, the ammonia remaining in this pipe andin the pipe 38 would go into solution and cause a suction in these pipesthereby drawing the solution up into the ammonia pipe 38. To preventthis, a by-pass pipe 52 is connected around, the air valve 40 and arestricted orifice 54 is disposed in said by-pass. The purpose of thisrestricted by-pass around the valve 40 is to provide for a continuoussmall flow of air into the tank bath from the aerator 28 when the valve40 is closed, thereby preventing the formation of a partial vacuum inthe aerator 28 and the pipe 34 as a result of the ammonia going intosolution.

If the temperature of the bath exceeds F., there will be an excessiveloss of the volatile constituents of the bath and there will befluctuations of the ammonia content of the bath. Accordingly, coolingmeans (not shown) may be provided to maintain a desired bathtemperature. However, the bath is still quite effective at temperaturesas high as F., but for a minimum loss of volatile constituents, the bathshould be maintained between 60 F. and 70 F. The temperature of the bathmay be maintained below 60 F. but the cost of cooling then becomesexcessive. In fact, the temperature of the bath may be reduced toapproximately 40 F. without any appreciable detrimental effect otherthan the increased cost of cooling, but if the temperature drops below40 F., some of the constituents of the bath will freeze out.

A fresh bath is made by dissolving ammonium carbonate (NI-102003 inwater. The permissible range of concentration of ammonium carbonate inthe bath is quite large and can vary from 2 ounces per gallon tosaturation. If the minimum concentration of 2 ounces per gallon is notmaintained, the time required to remove a given amount of copper becomesquite long. It should be noted that there may be some slight etching ofthe steel workpieces if they are allowed to remain in a bath whichcontains considerably less than this minimum concentration of 2 ouncesper gallon of ammonium carbonate. It is desirable to maintain a minimumammonium carbonate concentration of at least 8 ounces per gallon andpreferably the ammonium carbonate concentration should be at least 12ounces per gallon. This concentration is maintained by periodicallyadding ammonium carbonate to the bath. Excessive amounts of ammoniumcarbonate in the bath have no detrimental effect but if the ammoniumcarbonate concentration exceeds 16 ounces per gallon, there is a wasteof chemicals. Instead of periodically adding ammonium carbonate to thebath, it is possible to maintain its concentration therein byintroducing carbon dioxide CO2 and ammonia NH: into the bath.

It has also been found that the time required for the removal of a givenamount of copper plate is appreciably longer in a fresh bath of ammoniumcarbonate, as compared to a bath having a substantial copperconcentration, as exists in a bath which has previously been used.Accordingly, when a new bath is made up it is desirable also to use aportion of the old bath in order to provide the new bath with an initialcopper concentration. In addition, or in lieu of using a portion of anold bath, copper electrodes may be hung in the bath to helpbuild up aninitial copper concentration. The bath will operate with a range ofcopper concentration from 0 ounce per gallon to saturationthat is, toapproximately 20 ounces per gallon. However, a minimum copperconcentration of at least the 0.2 ounce per gallon is desirable in orderto increase the rate at which copper is removed from the copper-platedworkpieces. When the copper concentration exceeds 18 ounces per gab.

lon, there is a possibility of precipitating a copper ammonium compoundon the steel workpieces and the efllciency of the bath is reduced.Preferably, the range of concentration of copper in the bath should bebetween 0.5 and ounces per gallon.

The aforedescribed process, forv stripping cop per from steel parts, isvery simpleand does not require complicated apparatus. I'hus, theelectric and chemical control is verysimple since it is not necessary tomaintain the, electric current or the concentration of any ofthesjconstituents of the bath within any small orinarrow range. Anotheradvantage of the aforede'scribed process is that the copper removed-fromthe steel parts can readily be recovered 10m the bath as copper oxideCuO by heating said;,bath.

While I have described my invention in detail in its present preferredembodiment, it will be obvious to those skilled in thejar-t; afterunderstanding my invention, that various changes and modifications maybe made therein'without departing from the spirit or scope thereof. Iaim in the appended claims to coverall such modifications. I

I claim as my invention:

1. The process of removing copper from the surfaces of a pair ofmetallic bodi'estfsaid process comprising the steps of immersin'ggsaidbodies in a solution consisting of ammonium carbon-ate in water having0.5 to 15 ounces of copper dissolved therein per gallon of solution; andpassing an alternating current through said solution with said bodiescomprising opposite electrodes for said current, said alternatingcurrent comprising conventional symmetrical alternating current having afrequency of 60 cycles per second.

2. The process of removing copper from the surfaces of a pair ofmetallic bodies: said process comprising the steps of immersing saidbodies in a solutibn consisting of ammonium carbonate in water having0.5 to 15 ounces of copper dissolved therein per gallon of solution;passing an alterhating. current through said solution with said bodiescomprising opposite electrodes for said current, said alternatingcurrent comprising conventional symmetrical alternatinigj current havingaij frequency of cycles per second; and introdu'cing ammonia gas intosaid solution.

ROBERT L; PEASLEE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES nTE Ts OTHER REFERENCES Transactions of TheElectrochemical Society," vol. 7'1 1940) pages 207-209.

Metal Cleaning and Finishing, November 1933, pages 465, 466.

1. THE PROCESS OF REMOVING COPPER FROM THE SURFACES OF A PAIR OFMETALLIC BODIES: SAID PROCESS COMPRISING THE STEPS OF IMMERSING SAIDBODIES IN A SOLUTION CONSISTING OF AMMONIUM CARBONATE IN WATER HAVING0.5 TO 15 OUNCES OF COPPER DISSOLVED THEREIN PER GALLON OF SOLUTION; ANDPASSING AN ALTERNATING CURRENT THROUGH SAID SOLUTION WITH SAID BODIESCOMPRISING OPPOSITE ELECTRODES FOR SAID CURRENT, SAID ALTERNATINGCURRENT COMPRISING CONVENTIONAL SYMMETRICAL ALTERNATING CURRENT HAVING AFREQUENCY OF 60 CYCLES PER SECOND.